1. Field of the Invention
The present invention relates to a magnetic carrier to be contained in a developer which is used for electrophotography and an electrostatic recording method, a two-component developer having this magnetic carrier and a toner and an image forming method using a two-component developer.
2. Description of the Related Art
The process of developing an electrostatic charge image in electrophotography includes causing charged toner particles to adhere to the electrostatic charge image by utilizing an electrostatic interaction with the electrostatic charge image to thereby conduct image formation. A developer for developing an electrostatic charge image includes a one-component developer in which a magnetic substance is dispersed in a resin and a two-component developer in which a nonmagnetic toner mixed with a magnetic carrier is used. Particularly, the latter is preferably used in full color image forming apparatuses such as full color copying machines and full color printers in which high image quality is required. In addition, in late years, expansion of electrophotography into the field of print on demand (POD) brings about demands for high-speed printing ability and image printing, and as a result, high quality printed products having higher definition and no image defects have come to be desired.
Attempts to attain higher definition and higher image quality involve decreasing the toner particle size and decreasing the carrier particle size, and reducing the density and magnetic force of the carrier. A Mn—Mg ferrite is used in substitution for the conventional iron powder carrier or heavy metal ferrite so as to improve the quality and durability of the image in Japanese Patent Application Laid-Open No. 2000-330342. However, it cannot be said that the above-mentioned image quality desired in the POD print market is sufficiently satisfied, and further improvement in image quality and stability during extensive operation are needed. On this account, magnetic substance dispersion type resin carrier in which a magnetic substance is dispersed in a resin has been proposed for attaining a carrier having lower specific gravity and lower magnetic force. A magnetic substance dispersion type resin carrier having high electric resistance and lower magnetic force is proposed in Japanese Patent Application Laid-Open No. H8-160671. However, when such a carrier as mentioned above becomes lower in density and in magnetic force, it is possible to attain sufficiently high image quality and high definition and a further improvement in durability, but the developing characteristics may deteriorate. The cause of the deterioration in the developing characteristics is deterioration in the electrode effect, which is caused by higher resistance of the carrier. As a result, the toner at the end edge of the half tone part may be scratched off on the border between the half tone image part and the solid image part to cause white streaks and generate an image defect in which the edge of the solid image part is emphasized (hereinafter referred to as blank areas).
A resin filling type ferrite carrier in which pores of a carrier having a porosity of 10 to 60% which are filled with a resin is proposed in Japanese Patent Application Laid-Open No. 2006-337579 as a material substituted for the magnetic substance dispersion type resin carrier. Furthermore, a carrier having a structure in which pores of a porous ferrite core material are filled with a resin is proposed in Japanese Patent Application Laid-Open No. 2007-57943. These are low in specific gravity and strong against mechanical stress, and can produce a sufficient image density. They are excellent also in developing characteristics, and exhibit performance stable for a long period of time. However, in the field of POD where higher image quality is pursued, not only high developing characteristics and durability but also fewer image defects in each image on each sheet are required. Besides, as mentioned later, further improvement is necessary in the above carrier to obtain sufficient developing characteristics when the laid-on toner amount is reduced.
In addition, a carrier in which porous parts thereof are filled with a resin and the total volume of pores is defined and which uses a carrier core material having an electric resistance of 105 Ω·cm or more when a voltage of 500 V is applied is proposed in Japanese Patent Application Laid-Open No. 2007-218955. Japanese Patent Application Laid-Open No. 2007-218955 proposes a high-resistant carrier in which break-down is suppressed when a high voltage is applied. However, as mentioned above, when a carrier having an increased resistance is used, the developing characteristics may deteriorate and, as a result, image defects such as blank areas may be generated.
As a carrier which can provide clear black solid printing and inhibit carrier adhesion, a carrier which shows reversible and abrupt changes of electric resistance of 103 Ω·cm or more when the electric field intensity crosses the border of 1000 V/cm is proposed in Japanese notification Patent Gazette No. H07-120086. This carrier is one having a thin layer coating on a relatively low electric resistance carrier core particle. This carrier allegedly exhibits low electric resistance at high electric field intensity, which enhances the developing characteristics, while increasing the electric resistance of the carrier at lower electric field intensity, thereby inhibiting carrier adhesion. However, when a carrier which shows an abrupt change in electric resistance across the border of electric field intensity of 1000 V/cm is adopted in a developing apparatus to which an alternating electric field is applied, injection of electric charges may occur, which causes a roughened region in the half tone part. Besides, leaking of the electric charges may not be prevented, which may cause white spots.
It is necessary to achieve three factors, high speed, high image quality and low running cost, when an image forming method according to an electrophotographic method is contemplated to be applied to the field of POD. As characteristics expected of a toner, it is essential to achieve higher image quality and higher definition image as compared with the conventional level without narrowing a color gamut reproduction range. It is also essential to reduce toner consumption. Furthermore, it is necessary to reduce fixation energy and to adapt to various types of transfer materials.
Japanese Patent Application Laid-Open No. 2005-195674 makes a suggestion that problems such as blister are inhibited from occurring at the time of fixation while toner consumption is reduced by reducing the laid-on toner amount to 0.35 mg/cm2 or less, to thereby form high quality and high definition color images which stably have a wide color gamut reproduction range. According to this suggestion, it is allegedly possible to form high quality and high definition color images which stably have a wide color reproduction range, are reduced in roughening and are excellent in fixing properties. When a toner including toner particles having an increased amount of a coloring agent is used in a conventional electrophotographic system, a certain effect can be expected on the fixation characteristics, but the chroma of images may be lowered and the color gamut of images may be narrowed. The reason for these is presumed to be that as a result of increasing the amount of the coloring agent, the dispersion state of the coloring agent becomes worse, the hue is changed, the chroma of images is lowered, and the color gamut is narrowed.
As stated above, density stability and gradation characteristics are liable to deteriorate at the time of long-time use when the amount of a coloring agent contained in toner particles is increased. Conventionally, a toner has the γ-characteristics as shown by the curve A in FIG. 3 in which the horizontal axis represents the electric potential and the vertical axis represents the image density. In a conventional toner, when the content of a coloring agent is increased, it becomes possible to produce a given density on a transfer material with a smaller amount of laid-on toner and gradation characteristics are required to be produced with electric potential in a narrow range of developing contrast (see Japanese Patent Application Laid-Open No. 2005-195674). In this case, the γ-characteristics are as represented by the curve B of FIG. 3, where the slope of the γ-characteristics becomes steep, and high gradation characteristics are difficult to obtain in some cases. In addition, since the slope of the γ-characteristics is steep, a change in image density due to a change in electric potential increases as compared with a conventional toner, which may also decrease the gradation characteristics.
In the POD market, it is important that wide gradation characteristics can be obtained and color is stable, and so, it is preferable that development is conducted so that the γ-characteristics may have a gentle slope even with a small amount of laid-on toner. As a method for forming gradation with electric potential in the same developing contrast as before by using a toner having an increased content of the coloring agent, there is a method in which the frictional charge amount of the toner is increased. Japanese Patent Application Laid-Open No. 2005-195674 does not mention the frictional charge amount of the toner.
The developing characteristics and transfer characteristics may deteriorate and the image density may decrease when the frictional charge amount of the toner is increased since the electrostatic adhesive power to the surface of a carrier and the photosensitive member increases. In addition, as described above, it becomes a possible cause of the image defects such as blank areas. Japanese Patent Application Laid-Open No. 2006-195079 describes the relationship between the frictional charge amount of toner and the adhesive power between the toner and a carrier.
In Japanese Patent Application Laid-Open No. 2006-195079, the formation of high quality images free from image defects is enabled by setting each of the frictional charge amount of toner and adhesive power in a certain range. However, it is not stated that the content of a coloring agent in toner particles, which can reduce the consumption of the toner, is increased, and the frictional charge amount of the toner is adjusted in order to reduce the consumption of the toner, and the adhesive power between the carrier and toner is still so strong that sufficient image density may not be obtained.
On this account, in order to perform image formation with a smaller amount of laid-on toner than before, a carrier is needed which can efficiently carrying out development with a toner which has a high frictional charge amount, a large content of a coloring agent and strong coloring power and which is high in the dispersibility of the coloring agent.